In the field of electronic computers which is constantly striving for improvements in operational speed and performance, the silicon semiconductor devices used in most electronic computers have begun to reach a limit in operating speed and device size/packing density. Computer designers are always looking to reduce the switching time of switching circuits and to increase their packing density so as to permit the execution of the greatest number switching operations per unit time and yet reduce the space occupied by the switching circuits. Josephson devices have attracted keen attention in the past as one potential novel break-through toward satisfying these requirements, and thus logic circuits which incorporate these devices have been heavily studied.
The use of Josephson junctions as electronic circuit elements only became possible after 1962 when the Josephson effect was discovered. At that time the only known superconducting materials had such low critical temperatures (i.e., temperatures below which the material becomes superconducting) that any circuit using Josephson junction elements required a source of liquid helium to maintain the low temperature required. Since that time the improvements in low-temperature technology, and the discovery of high T.sub.c superconducting materials, have made these limitations less sedous, but other practical considerations still plague commercialization of superconducting logic circuits.
Josephson junctions are highly-unusual, nonlinear circuit elements which can be used to design circuits having many interesting properties. The primary advantages of these junctions are said to be their low power requirements and high operating speeds compared with conventional, nonlinear circuit elements. That is, the Josephson device exhibits high-speed switching operations with low power dissipation and high sensitivity in accordance with the Josephson effect produced in a superconductive state at very low temperatures. Therefore, the Josephson device has, in the past, given rise to expectations for materialization of super-high speed computers.
The Josephson device, in its basic construction, comprises two superconductors joined to each other through the medium of a thin insulating film (Josephson tunneling junction) as typified by the Josephson tunnel junction device. In this construction, when the current supplied to the junction exceeds the junction's critical current, the device is transferred from the zero-voltage state to the voltage state (a switching operation). That is, the operation of the device is based on the existence of two states for the gate (Josephson junction) and the fact that the gate can be switched from one state to the other by means of a magnetic field or applied current. One of these states is a pair tunneling state of the junction in which current can flow through the barrier region without any voltage drop. The other state is a single-particle tunneling state in which the current flows with a voltage across the junction equal to 2 Delta, where Delta is the energy gap of the superconductor. For tin, 2 Delta equals about 1 mV at 1.7 K. The transition from one state to the other can be brought about by exceeding the critical current for the Josephson junction. The critical current, I.sub.j, is defined as the largest zero voltage current the junction can carry. Therefore, the device's I-V characteristics are such that the voltage across the junction remains at zero until the current reaches a critical value at which time the voltage across the device jumps to a finite value and thereafter varies slowly with further increase in current. The critical current value is dependent upon the magnetic field applied to the Josephson device.
U.S. Pat. Nos. 3,626,391; 3,281,609; 3,758,795; 3,825,906, etc. disclose the concept of utilizing a Josephson junction for Josephson circuit devices as, for example, a memory cell, logic gate or shift register.
There are two basic types of Josephson logic gates, current injection gates and magnetically controlled gates. All present day Josephson integrated circuits utilize one or both of these gate types. The fastest logic gates utilize a combination of the two. All of these types of Josephson integrated circuits are subject to the disadvantages described hereinbelow.
Conventional superconductive logic circuits are roughly classified by the input system into the magnetic coupling type and the current injection type. In a superconductive logic circuit of magnetic coupling type, an input signal is magnetically coupled to a loop including a Josephson junction and inductance, and logic operations are performed by transformation of the Josephson junction to the non-zero voltage state as shown in U.S. Pat. No. 3,978,351. In this example, the Josephson junction and the inductance constitute a magnetic flux quantum interferometer, and the product of the inductance L and the and the critical current I.sub.j of the Josephson junction is selected to be close to one magnetic flux quantum. Therefore, when the critical current I.sub.j is made small for energy consumption, a large inductance L is required, making it difficult to realize a compact circuit and reducing the operating speed. Conversely, when the inductance L is made smaller for obtaining a higher operating speed, the value of the critical current I.sub.j becomes greater and energy consumption increases. Further, the circuit is subject to the influence of external magnetic noise, stray inductance and so on, resulting in extreme fluctuations and unstable operation. Such a circuit is also defective in that uniform and efficient connection of a number of input wires is structurally difficult.
A superconductive logic circuit of current injection type has been an improvement in that it does not involve a magnetic flux quantum interferometer. In a superconductive logic circuit of current injection type, current is directly supplied to the Josephson junction for switching into the non-zero voltage state in order to perform logic operations. A prior art superconducting logic circuit of the current injection type which does not involve a magnetic flux quantum interferometer is shown in IEDM "Josephson Direct Coupled Logic (DCL)" (1492, 12), IBM. According to this example, the defects of the superconductive logic circuit of the magnetic coupling type which involve magnetic flux quantum interferometer are solved. However, the threshold for determining the sensitivity is solely determined by the switching of the non-zero voltage state of a single Josephson junction so that only a current gain of at most 1 may be obtained. Therefore, although it is advantageous to use it as a switch, it is difficult to apply it to various kinds of logic circuits.
In an article entitled "Threshold Logic" by Daniel Hampel and Robert Widner, published in IEEE Spectrum, May, 1971, pp. 32-39, threshold logic gates and means for implementing such gates with large scale integrated circuitry are disclosed. As pointed out in the article, threshold logic gates have increased logic power over standard Boolean logic gates such as AND, OR, NOR gates. Basically, a threshold logic gate receives N logic inputs, weights the N inputs either equally or with unequal weights, sums the weighted inputs, and provides a logic output if the sum is greater than or equal to a threshold weighting factor.
Conventional threshold logic is implemented by using either of the current sources and a threshold detector, or magnetic flux summing techniques described above. Current summing techniques and magnetic flux summing techniques require precision in generating the analog quantity which will be compared to the threshold. Precision is also required when Josephson devices are used. The source of analog precision is provided in the Josephson case by voltage referred to above as Delta. The parameter Delta is essentially the gap in the energy spectrum of the conduction electrons of the superconductor being considered and as such is a material constant.
Logic circuits incorporating Josephson junctions have other disadvantages in addition to those discussed above. For instance, some switching circuits in Josephson junction technology have the severe disadvantage of not being automatically resetting, thus requiring additional switches for their reset operation. While this would not pose a technical problem, the economics of any device incorporating the state of the art switching circuits must suffer considerably through long cycle times. Another disadvantage is the problem of eliminating cross-talk between Josephson devices in Josephson logic arrays, which are due to current transients when a device switches. Also, the current-voltage characteristic of a Josephson tunnel junction device is known to have an unstable region at low voltages. If one attempts to voltage bias the device in the unstable region, its operating point jumps back and forth between the supercurrent state (V=0) and the finite voltage state (V not=0, typically V=2 DELTA, the gap voltage of the superconductor). This problem is known as relaxation oscillation.
Therefore, the conventional Josephson device has been unable to simultaneously satisfy three conditions, i.e., (1) the size reduction of the device which permits integrated circuits in high density, (2) the high sensitivity which produces wide operation margin, and (3) perfect isolation between the input and output signal currents in the device. The three conditions are indispensable to the components of future electronic computers to obtain stable, high-speed logic circuit operation.
Another disadvantage of Josephson junction technology is the large number of processing steps required to produce the devices. A primary factor determining the efficacy of integrated circuit processes and the concomitant yield thereof is the number of steps comprising the process. For example, if a process consists of twelve steps and the expected yield of each of the steps is ninety percent, then the yield of operative devices at the completion of the twelve step process is 0.9.sup.12, or approximately 28%. If, however, the process consists of eight steps, each with a yield probability of ninety percent, then the final yield for the eight step process is 0.9.sup.8, or 43%. Thus by eliminating steps, an improvement in yield is achieved without any improvement in the quality of the processing. Additionally, large numbers of processing steps engender problems with adhesion, step coverage and damage to prior deposited layers. The longer the fabrication sequence, the lower is the device throughput of the process.
Josephson junction logic integrated circuit fabrication involves approximately 12 deposition steps, 12 photoresist steps, an anodization step and a junction barrier formation step. Typically the process comprises depositing four superconducting layers, viz., the ground plane, the lower Josephson electrode, the Josephson counter electrode and the control lines. Interconnections, interferometer loops and other circuit elements are formed from the last three layers. Each superconductive layer is separated from an adjacent superconductive layer by an insulator layer that is patterned to form vias which provide required electrical connections between layers. The deposition of resistors, additional insulator layers for increased inductance, passivation layers and anodization are steps utilized to complete the circuit. Thus it is appreciated that a minimum of nine separate thin films and patterning steps are required in this process. Therefore a reduction in the number of processing steps required to form the logic circuits is desirable.
A switching technology which is similar to Josephson junction technology and which has been around for 30 years is the Ovonic threshold switch. This device starts in the "off" or non-conducting state and a critical voltage is required to switch it on. Its I-V characteristic looks like that of the Josephson junction, but the current and voltage axes are transposed. Its switching speed, like that of the Josephson junction, is also limited by the device capacitance, but since the devices are thicker, it exhibits a lower capacitance for a given lithography. Additionally, the speed/power potential of the Ovonic Threshold Switch compares favorably with the silicon and gallium arsenide technologies as well as the practical implementations of Josephson logic.
Now that the end of the dramatic density increases in silicon technology is at hand, the real need in advanced logic is to find a superior future technology which can replace silicon transistors.
Therefore the object of this invention is to define a novel logic family which employs chalcogenide Ovonic threshold switches as the logic gates therein.